Conventionally, aqueous solution batteries, such as a lead battery, a nickel-cadmium battery, or a nickel-hydrogen battery, have become mainstream in the field of a rechargeable secondary battery. Recently, lithium secondary batteries having a high energy density have attracted increasing attention with reduction in size and weight of electric devices, and thus have been studied, developed, and commercialized rapidly. At present, small-sized consumer-oriented lithium secondary batteries are widely used for a cellular phone and a laptop computer.
On the other hand, electric vehicles (EV) and hybrid electric vehicles (HEV) designed to assist part of driving power using an electric motor have been developed by various automakers from the viewpoint of problems, including global warning and fuel depletion. Thus, a secondary battery with high output power and capacity is required as a power source for such vehicles. Nonaqueous solution lithium secondary batteries having a high voltage have received attention as the power source for satisfying such a requirement.
However, even the lithium secondary battery having a high voltage is required to achieve large current charge and discharge in addition to the high voltage, for example, because the motor for driving the HEV has high output. Thus, each electrode of the battery for the HEV includes a portion not having an active material mixture layer, namely an active material mixture layer-free portion to be apart of a current collector in addition to an active material mixture layer. The active material mixture layer portion and the active material mixture layer-free portion are arranged on a surface of metal foil in a width direction of metal foil corresponding to an axial direction of an assembly and extended in a length direction of the metal foil lying at right angles to the width direction. The active material mixture layer free-portion is arranged at one end side in the width direction to be a part of collector so as to enable the large current charge and discharge. The active material mixture layer free-portion is used as a lead to be directly or indirectly connected to an external current collector terminal, thereby improving a current collecting efficiency (see, for example, JP-A-2005-268139).
However, in the electrode including the active material mixture layer portion and the active material mixture layer free-portion, a metal foil (collector foil) portion where the active material mixture layer portion is positioned is although rolled by press working for adjusting a density of the active material mixture layer, a metal foil (collector foil) portion where the active material mixture layer-free portion is positioned is not rolled. As a result, a shape of the electrodes become distorted, which may cause positions where the positive electrode mixture layer and the negative electrode mixture layer are not opposed to each other via the separator due to the degree of distortions of the electrodes in manufacturing the electrode assembly. The positions where the mixture layers of both electrodes are not opposed to each other are difficult to be subjected to the charge and discharge reaction. The existence of such positions of the mixture layers not subjected to the reaction may disadvantageously apply the excessive charge and discharge reaction to other remaining positions of the mixture layers opposed to each other, which leads to reduction in capacity and lifetime of the battery due to degradation of active material. Further, the nonuniform reaction may generate lithium dendrite to cause internal short circuit.
The present invention is to solve such problems and to provide a lithium secondary battery which enables to charge and discharge for large current with excellent charge and discharge characteristics and safety.